For the manufacturing of a backside metallization of a solar cell, generally an aluminum backside paste is printed onto the back side of the solar cell by screen printing and then the solar cell with the printed-on aluminum backside paste is heated. The aluminum backside paste is highly porous, in order to avoid bending of the solar cell during cooling after heating because of the different coefficients of thermal expansion of the aluminum of the backside paste and of the semiconductor material of the solar cell. The porosity of the aluminum backside paste (even after printing on and heating) has the effect of further impairing the in any case already poor mechanical properties of the aluminum; thus, for example, the low yield stress of the aluminum decreases even further.
Because of the poor mechanical properties of the aluminum backside paste (or of layers that consist of this paste), many direct connecting technologies of typical cell connectors or else wires on aluminum layers that have been printed with such a conventional aluminum backside paste fail, even though the connection between the cell connector and the porous aluminum layer itself is very stable, because the printed-on aluminum layer cohesively tears within itself.
Consequently, a mechanically stable aluminum layer cannot be applied to a solar cell by printing processes when a conventional aluminum backside paste is used, because it either leads to excessive bending or rupture of the solar cell during cooling or has poor mechanical properties owing to its porosity, which is necessary in order to prevent the mechanical stresses.
For these reasons, in order to establish electrical contact on the back side of a solar cell, silver-containing busbars are usually printed onto the back side of a solar cell. However, silver is expensive (and, what is more, increasing prices and a shortage of silver can be expected in the future) and, furthermore, no electrical backside field (back surface field—BSF) can be formed in the regions in which a silver-containing busbar is printed on. This leads to a reduction in the efficiency of the solar cell. Even the printing on of what are known as soldering pads instead of continuous busbars cannot entirely obviate the need for silver to be used nor can a continuous electrical back surface field be provided in this way.
DE 10 2009 034 594 A1 discloses a method for manufacturing a crystalline silicon solar cell with full-area, alloyed backside metallization. According to DE 10 2009 034 594 A1, a thin aluminum layer is sputtered or vapor-deposited onto the entire back side of a solar cell. Then, backside busbars are printed on using a silver or silver-aluminum paste and after that the region between the busbars including silver is printed with a thicker layer of aluminum screen-printing paste. This method has the disadvantage that, on account of the need for silver and the expenditure required for the method step of applying the thin aluminum layer, it is costly.